1. Field of the Invention
This invention relates to an equipment used in semiconductor fabrication, and more particularly to a dispensing system for a spit-on glass (SOG) coater.
2. Description of Related Art
In fabrication processes a wafer often has a dielectric layer with uneven contours on top. If the uneven contours are not properly planarized, it may cause a difficulty to form an interconnect on it and cause a loss of pattern transferring precision. A planarization process to planarize the wafer surface is therefore greatly needed in semiconductor fabrication.
Currently, a spin-on glass (SOG) technology is widely used to locally planarize the wafer surface. The SOG technology includes a solvent containing a dissolved di-electric material. Such as silicon dioxide, and coats the wafer surface with the solvent by a spin-coating, process so as to fill concave regions on the wafer surface. A purpose of local planarization is then achieved. A complete SOG process needs two steps: coating and curing. The coating step is to distribute solvent to cover the wafer surface, and the curing step is to vaporize residual solvent by a thermal process so as to increase the SOG density and cure the structure of silicon oxide.
In the SOG process, a spin coater, such as DSN 60A, is usually used for spin-coating the wafer. FIG. 1 is a block diagram of a conventional solvent dispensing system used in a spin coater. FIG. 2 is an equipment structure of the dispensing system in FIG. 1. In FIG. 1 and FIG. 2, the solvent dispensing system includes a solenoid valve 10, a sucking-back valve 12, a switch valve 14, a pressure regulating valve 18, and several speed controllers SC1, SC2, SC3, and SC4. A solvent 16 is contained in a container 17, which can be pressurized.
A solvent flowing route is first described here. A pressure is created in the container 17 through the pressure regulating, valve 18 so as to drive the solvent 16 to the switch valve 14. When switch valve 14 is open, the solvent 16 flows through the sucking-back valve 12 ard reaches to a dispensing end 27. The solvent 16 thereby is dispensed on to a wafer 20, which is held by a spinner 22.
The switch valve 14 and suckinig-back valve 12 are further controlled by the solenoid valve 10 through the speed controllers SC1, SC2, SC3, and SC4. Air is used as an intermedium for sending control signals from the solenoid valve 10. The speed controllers SC1 and SC2 forming together as a speed control unit 13 are coupled in series between the switch valve 14 and the solenoid valve 10. The speed controllers SC3 and SC4 forming together as a speed control unit 15 are coupled in series between the sucking-back valve 12 and the solenoid valve 10. The solenoid valve 10 controls both the switch valve 14 and the sucking-back valve 12 through air, which serves as an intermedium for sending control signals from the solenoid valve 10. The solenoid valve 10 includes one air-in (AIR) end to engulf air, and one air-out (EXH) end to release air. Through the AIR end and the EXH end, the solenoid valve 10 can control the switch valve 14 and the sucking-back valve 12.
Each of speed controllers SC1, SC2, SC3, and SC4 is identical. The speed controller SC1 used as an example includes a regulating valve 26 and an one-way valve 24. The directions of the one-way valves 24 in the SC1 and the SC2 are opposite to each other.
The solenoid valve 10 can switch the dispensing system in two operation modes; a dispensing mode and a stopping mode. In the dispensing mode, the solenoid valve 10 switches the system from a stopping status to a dispensing status, and in the stopping mode, the solenoid valve 10 switches the system from a dispensing status to a stop status. Ideally, when the solenoid 10 switches to the stopping mode, the switch valve 14 is necessary to be first closed and then the sucking-back valve 19 is activated to suck the solvent 16 a little back from the dispensing end 27 in order to prevent the solvent 16 from dropping onto the wafer 20.
However, it practically is very difficult to have this time order. If the sucking-back valve 12 is activated before the switch valve 14 is closed, the suck effect is invalid. This may result in a few undesired solvent dropping onto the wafer 20 if there is any disturbance on the solvent 16 at the duct end 27. This causes a damage on the wafer 20. Similarly, when the system is switched to a dispensing mode, if the sucking-back valve 12 is firstly switched off to release the sucking force and the switch valve 14 is secondly switched on to supply the solvent 16, then again a few undesired solvent drops may drop onto the wafer 20 during the transition period. This also causes a damage on the wafer 20. A dispensing rate of the solvent 16 may also unstable.